1. Source of the Invention
This invention was made with support under Dow Chemical Company Contract No. 443120-57276-3. Dow Chemical Company may have some rights in this invention.
2. Field of the Invention
The present invention relates to the synthesis of novel lithium aminoborohydrides, and to their use of a powerful selective reducing agents for organic compounds.
3. Description of Related Art
A commercially safe, economical reducing agent has been the goal of a number of acedemic and industrial groups. Some art of general interest is as follows:
C. T. Goralski et al., U.S. Pat. No. 4,886,924. PA1 C. T. Goralski et al., U.S. Pat. No. 4,895,996. PA1 H. C. Brown et al. (1988) Accounts for Chemical Research, Vol. 21 (#8), p. 287.
More specific references are described below and are referred to in the subsequent text as (Ref. 2) or (2a), etc.
References of interest include:
1. (a) E. R. H. Walker, (1976) Chem. Soc. Rev., Vol. 5, p. 23;
(b) A. Hajos, (1979) Complex Hydrides, Elsevier: N.Y.
2. (a) H. C. Brown et al., (1956) J. Am Chem. Soc., Vol. 78, p. 3616;
(b) H. C. Brown et al., (1953) Ibid., Vol. 73,p. 6263;
(c) C. A. Brown et al., (1973) J. Chem. Soc. Chem. Comm., 391.
3. J. H. Golden et al. (1992), Inorg. Chem., Vol. 31, 0000. 4. H. C. Brown et al. (1984), J. Org. Chem., Vol. 49, p. 885.
5. (a) E. R. Garret et al. (1953), J. Am. Chem. Soc., Vol. 73, p. 6051;
(b) H. Habenstock et al. (1962), Ibid., Vol. 84, p. 2368.
6. A. L. Allred et al. (1958), J. Inorg. Nucl. Chem., Vol. 5, p. 264.
7. F. A. Davis et al. (1971), J. Org. Chem., Vol. 36, p. 1300.
8. R. O. Hutchins et al. (1984), J. Org. Chem., Vol. 49, p. 2438.
9. H. C. Brown et al. (1982), Organomet. Chem., Vol. 239, p. 43.
10. B. Singaram et al. (1991), J. Org. Chem., Vol. 56, p. 5691.
11. Hutchins et al. (Ref. 8) report a .sup.11 B-NMR chemical shift value of .delta.+43 for sodium dimethylaminoborohydride. The values obtained in our study differ considerably from that reported by Hutchins et al.
All patents, patent applications, articles, references, standards, cited in this application are incorporated by reference.
Substitution of one or more of the hydrogen atoms on a borohydride with electron donating groups, such as alkoxy or alkyl groups, has attracted considerable interest over the past decade as a method of fine-tuning the hydride delivering ability of the borohydride moiety (Ref. 1). Although borohydrides with three alkoxy groups are known (Ref. 2), borohydrides with one or two alkoxy groups are at best fleeting intermediates (3). Trialkoxyborohydrides are very mild reducing agents (4). This result is unexpected because hydride transfer would be expected to be more difficult from a stronger Lewis acid, such as BH.sub.3, than from a weaker Lewis acid, such as (RO).sub.3 B, a compound weakened by back bonding (5). The inductive effect of the alkoxy group may predominate over the mesomeric effect. Monoalkoxy substituted borohydrides may, therefore, be even better reducing agents. However, no monoalkoxyborohydrides have ever been reported.
One type of heteroatom-substituted borohydride that potentially may have useful characteristics are the aminoborohydrides. Because of the lower electronegativity of nitrogen compared to oxygen (3.07 vs. 3.50) (6), better donation of the lone pair of electrons toward the boron atom from the nitrogen atom may be possible. Consequently, aminoborohydrides might have enhanced hydride delivering ability compared to either borohydride or the trialkoxy derivative. Moreover, aminoboranes are weaker Lewis acids than BH.sub.3 or (RO).sub.3 B (7). These properties suggest that once produced aminoborohydrides should be better reducing agents. This result was demonstated recently by the synthesis of sodium dimethylaminoborohydride and the study of its reduction characteristics (8). The aminoborohydride was prepared by the reaction of sodium hydride with dimethylamine borane (e.g. reaction sequence 1) (Ref. 8). ##STR1##
Sodium dimethylaminoborohydride showed enhanced reducing ability. Thus, it not only reduced aldehydes and ketones to corresponding alcohols, but was also found to be an effective reagent for the conversion of esters to alcohols and primary amides to amines. Tertiary amides were reduced to either alcohols or amines, depending on the steric requirements of the amide group. It is known that the reactivity of the borohydrides decreases rapidly in the series lithium borohydride&gt;sodium borohydride&gt;potassium borohydride (9). Consequently, lithium aminoborohydrides, when obtained, would be expected to be more powerful reducing agents than the corresponding sodium derivatives.
At this time, the synthesis of novel lithium aminoborohydrides has not been described. Further, the use of these novel hydrides as powerful reducing agents has not been reported. The present invention provides a synthesis of lithium aminoborohydrides and their use as powerful reducing agents.